Reduce Blocking Artifacts Research Articles

A directional deblocking filter based on intra prediction for H.264/AVC

Although the adaptive deblocking filter of H.264/AVC efficiently reduces blocking artifacts for most images, it is limited in removing the blocking artifacts of the directional edge on the block boundaries. In order to solve this problem, we propose a directional deblocking filter based on intra prediction direction. The proposed algorithm performs directional filtering according to the direction of the intra prediction mode in order to correctly find the block edge while avoiding computational burden. Experimental results show that the proposed algorithm provides subjective improvements at the directional edge and PSNR gain of up to 0.13dB.

Blind noise estimation for blocking artifact reduction in discrete cosine transform coded images

We present blind noise strength estimation for removing blocking artifacts associated with block-based compression schemes. The proposed framework is based on edge statistics residing across two neighboring blocks. First, the block boundary discontinuity is statistically analyzed. Next, the discontinuity is approximated by a simple model, and then filtered by controlling a filter strength according to the contents of the blocks. The main advantages of the proposed algorithm are that 1. it is a blind noise estimator without any prior information, 2. it models a noise with the combination of block discontinuity and simple noise-shape components, and 3. it adjusts the strength of a filter adaptively for removing the given noise. To evaluate the proposed scheme, several baseline approaches are described and compared in terms of peak signal-to-noise ratio and a quality metric.

Image fitting using arctan for JPEG AC coefficient prediction

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We explore predictive coding for the JPEG baseline sequential algorithm in this article. Our objective is to reduce inter-block redundancy of discrete cosine transform (DCT) blocks. We first present an implementation of image interpolation approach and develop a method for predicting ac coefficients of a block in terms of sum of pixels (or dc coefficients) of some surrounding patches. If we use 2-D quadratic polynomials for interpolation over 24 <formula formulatype="inline"><tex>$\times$</tex></formula> 24 pixel area, our method derives the same equations, which JPEG developed for reducing blocking artifacts. Next, we show that nonpolynomial functions such as <formula formulatype="inline"> <tex>$\arctan$</tex></formula> perform better for ac prediction. We develop additional prediction equations considering different configurations of the interpolation area. Finally, we present performance results, which show that for images compressed at the default quality factor of 75, our equations reduce the ac energy of DCT blocks by about 22% and the final code bit size by about 5.5% over the JPEG sequential algorithm. At the quality factor of 90, the code size reduction is 10.8%. We also present an adaptive dc coding method by considering the prediction algorithm of JPEG-LS. It reduces the average JPEG dc residual by over 20% and the dc code bits by 7.75%. </para>

Hybrid Temporal Error Concealment Methods for Block-Based Compressed Video Transmission

Boundary matching algorithm (BMA) and decoder motion vector estimation (DMVE) are two well-known temporal error concealment methods using the matching-based approach. In these two methods, the motion vector of each missing block is estimated by choosing one among candidate motion vectors which minimizes a sum of absolute differences (SAD) between boundary pixels of the corrupted macroblock. In general, the performance of DMVE is better than that of BMA. However, depending on the location or pattern of the corrupted block, BMA produces higher visual quality than DMVE. In this paper, we propose two types of hybrid error concealment methods; switching method and blending method. The switching method chooses one of two results obtained by BMA and DMVE based on the normalized SAD values. In the blending method, the weighted sum of the results concealed by the aforementioned two methods is utilized to improve the performance of error concealment. In order to reduce blocking artifacts further, the modified overlapped-block motion compensation is adaptively applied to the concealed blocks. Simulation results show that the proposed methods outperform other techniques in terms of subjective visual quality as well as PSNR performance.

Quality Enhancement for Motion JPEG Using Temporal Redundancies

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The paper proposes a pixel-based post-processing algorithm to enhance the quality of motion JPEG (MJPEG) by exploiting the temporal redundancies of the decoded frames. The technique permits reconstruction of the high frequency coefficients lost during quantization, thereby reducing ringing artifacts. Based on the linearization of the quantization function, the error between the estimated and original coefficients is analyzed for both cases of ideal and real video sequences. Blocking artifact reduction is verified by a reduction in the variance of this coefficient error. The condition of valid motion vectors to get quality improvement is considered based on these errors. The algorithm is also extended to find the optimal filter for a general estimation scheme based on an arbitrary number of frames. Results in visual and peak signal-to-noise ratio improvement using both integer and subpixel motion vectors are verified by simulations on video sequences. </para>

Transmission of modified JPEG images through AWGN channel

Joint Photographic Experts Group (JPEG) is a standard for compressing continuous-tone images. In this paper, image is compressed based on JPEG, but in place of Huffman coding, Run Length Encoding (RLE)/Vector Quantisation (VQ) has been applied and performance evaluated. RLE does not require table storage, but produces lesser compression than Huffman. VQ achieves higher compression at the cost of reduced quality. Further, the reconstructed image has been filtered to reduce blocking artifacts. When the encoded image is corrupted with Additive White Gaussian Noise (AWGN), VQ is more robust than Huffman/RLE since it is fixed-length encoding technique.

Optimization of Deblock Filtering Module Ported on OMAP1510

The Open Multimedia Applications Platform (OMAP) architecture provides a system solution for the wireless market. This work discusses the implementation of a part of ISO/IEC 14496, which was developed in response to a growing need for a coding method. To reduce blocking artifacts as a result of quantization in decoded video stream the deblock filter is applied as a post processing operation. This paper discusses Memory and million instructions per second (MIPs) optimization of deblock filtering module ported on Open Multimedia Applications Platform (OMAP 1510), and presents the results.

Subjective and Objective Comparison of Advanced Motion Compensation Methods for Blocking Artifact Reduction in a 3-D Wavelet Coding System

We compare, both objectively and subjectively, the performance of various advanced motion compensation methods, including overlapped block motion compensation (OBMC) and control grid interpolation (CGI), in a 3-D wavelet-based coding system. The motion vectors of the methods are obtained by using a sequence of 1-D dynamic programming algorithms that minimizes the cost function. Our experiment results indicate that an OBMC sequence usually has a higher peak signal-to-noise ratio (PSNR) than other methods, while a CGI sequence usually contains the fewest blocking artifacts. We provide a simple framework that combines the OBMC and CGI sequences. The proposed hybrid method removes more than 50% of the blocking artifacts of an OBMC sequence, while simultaneously maintaining a high PSNR performance.

Frame rate up-conversion using perspective transform

In this paper, we propose a frame rate up-conversion (FRUC) algorithm to increase the temporal resolution of video sequences at the decoder side. First, the proposed algorithm segments a frame into several objects. Then, perspective transforms are used to motion-compensate each object, while the translational block matching algorithm (BMA) is applied to the background. Furthermore, the overlapped block motion compensation (OBMC) technique is used to reduce blocking artifacts in boundary blocks. Experimental results show that the proposed algorithm provides better performance than the conventional approach.

Lapped-orthogonal-transform-based adaptive image watermarking

A robust, invisible watermarking scheme is proposed for digital images, where the watermark is embedded using the block-based lapped orthogonal transform (LOT). The embedding process follows a spread spectrum watermarking approach. In contrast to the use of transforms such as discrete cosine transform, our LOT watermarking scheme allows larger watermark embedding energy while maintaining the same level of subjective invisibility. In particular, the use of LOT reduces block artifacts caused by the insertion of the watermark in a block-by-block manner, hence obtaining a better balance between invisibility and robustness. Moreover, we use a human visual system (HVS) model to adaptively adjust the energy of the watermark during embedding. In our HVS model, each block is categorized into one of four classes (texture, fine-texture, edge, and plain-area) by using a feature known as the texture masking energy. Blocks with edges are also classified according to the edge direction. The block classification is used to adjust the watermark embedding parameters for each block.

Novel techniques for reducing blocking artifacts in motion compensated frame

We present two new techniques for reducing blocking artifacts in motion compensated frames. These techniques can be applied generally to motion compensated frames obtained by employing any partitioning schemes. In the first technique, the weighting coefficients of the overlapped block motion compensation are modeled with fuzzy relations. These fuzzy relations are formed based on two fuzzy sets, which define the closeness of a given pixel to the block boundaries in the horizontal and vertical directions, respectively. The second technique uses the similarity between a set of pixels inside a partitioned region and that on the borders to modify the border pixels. Simulations conducted on several benchmark video sequences indicate that the proposed techniques can improve the subjective quality of the frames as well as reduce the prediction error.

Motion-compensated frame rate up-conversion based on block matching algorithm

In this paper, we propose a frame rate up-conversion (FRUC) method using the motion compensation (MC) based on block matching algorithm (BMA). The property of the BMA is changed by the size of block. It is desirable for small moving object to set block size small. On the other hand, we would like to set the block size large in the global motion region. Therefore, the block size is decided depend on the property of the local region of frame image. In this paper, we present a novel FRUC technique based on the motion compensated interpolation with multi-size blocks. At first, the frame image is divided into 8x8 blocks and motion vectors are estimated for each 8x8 block. Next, the similar motion blocks are merged into one block, up to 32x32 block. Thus the proposed method reduces block artifacts and realizes the clear interpolation of moving objects regardless of the size of those.

Block artifact reduction using a transform-domain Markov random field model

The block-based discrete cosine transform (BDCT) is often used in image and video coding. It may introduce block artifacts at low data rates that manifest themselves as an annoying discontinuity between adjacent blocks. In this paper, we address this problem by investigating a transform-domain Markov random field (TD-MRF) model. Based on this model, two block artifact reduction postprocessing methods are presented. The first method, referred to as TD-MRF, provides an efficient progressive transform-domain solution. Our experimental results show that TD-MRF can reduce up to 90% of the computational complexity compared with spatial-domain MRF (SD-MRF) methods while still achieving comparable visual quality improvements. We then discuss a hybrid framework, referred to as TSD-MRF, that exploits the advantages of both TD-MRF and SD-MRF. The experimental results confirm that TSD-MRF can improve visual quality both objectively and subjectively over SD-MRF methods.

Reduction of blocking artifacts in JPEG compressed images

Image compression is a very important issue for several applications in the area of multimedia communications, the objective being reduction of storage and transmission costs. Many efficient coding techniques have been developed for various applications. Amongst them, the Joint Photographic Experts Group (JPEG) has been recommended for compression of continuous tone still images. However, the reconstructed images from JPEG compression produce annoying blocking artifacts near block boundaries, particularly in highly compressed images, because each block is transformed and quantized independently. Several techniques/algorithms have been proposed by researchers, both in spatial and frequency domains, for reduction of these artifacts with varied degree of success. These are briefly overviewed here. A new technique working in frequency domain, is proposed here by authors. This paper puts forth a method and an algorithm, working in frequency domain, for the detection and reduction of such blocking artifacts. These artifacts are modeled here as 2-D step functions between two neighboring blocks. Presence of the blocking artifacts is detected by using block activity based on human visual system (HVS) and block statistics. The boundary regions between blocks are identified as either smooth or non-smooth regions. The blocking artifacts in smooth regions are removed by modifying a few DCT coefficients appropriately, whilst an edge-preserving smoothing filter is applied to the non-smooth regions, i.e., genuine edges. The algorithm has been applied to variety of JPEG compressed images and results are compared with other postprocessing algorithms. The reduction in the blocking artifacts for each image have been evaluated using three indices, namely peak signal-to-noise ratio (PSNR), mean structure similarity (MSSIM) index based on human visual perception, and a new index, called here block boundary measure (BBM), applied to both vertical and horizontal block boundaries. The results show that the proposed method is very effective in detecting and reducing the blocking artifacts in JPEG compressed images.

A smoothness constraint set based on local statistics of BDCT coefficients for image postprocessing

In blocking artifacts reduction based on the projection onto convex sets (POCS) technique, good constraint sets are very important. Until recently, smoothness constraint sets (SCS) are often formulated in the image domain, whereas quantization constraint set is defined in the block-based discrete cosine transform (BDCT) domain. Thus, frequent BDCT transform is inevitable in alternative projections. In this paper, based on signal and quantization noise statistics, we proposed a novel smoothness constraint set in the BDCT transform domain via the Wiener filtering concept. Experiments show that POCS using this smoothness constraint set not only has good convergence but also has better objective and subjective performance. Moreover, this set can be used as extra constraint set to improve most existing POCS-based image postprocessing methods.

A deblocking method for BDCT compressed images based on adaptive projections

A deblocking method based on projection onto convex sets (POCS) is proposed to reduce blocking artifacts in compressed images coded by the block discrete cosine transform. The method differs from existing POCS-based methods in three aspects. Firstly, the adjustment of a pixel's intensity is determined by local properties of the pixel. Secondly, three locally adaptive constraint sets are introduced to improve deblocking results. Thirdly, the human visual system modeling and relaxed projections are incorporated to make pixels adjust appropriately. The method is tested on typical images with excellent results.

Regression-based prediction for blocking artifact reduction in JPEG-compressed images

In order to reduce the blocking artifact in the Joint Photographic Experts Group (JPEG)-compressed images, a new noniterative postprocessing algorithm is proposed. The algorithm consists of a two-step operation: low-pass filtering and then predicting. Predicting the original image from the low-pass filtered image is performed by using the predictors, which are constructed based on a broken line regression model. The constructed predictor is a generalized version of the projector onto the quantization constraint set, or the narrow quantization constraint set. We employed different predictors depending on the frequency components in the discrete cosine transform (DCT) domain since each component has different statistical properties. Further, by using a simple classifier, we adaptively applied the predictors depending on the local variance of the DCT block. This adaptation enables an appropriate blurring depending on the smooth or detail region, and shows improved performance in terms of the average distortion and the perceptual view. For the major-edge DCT blocks, which usually suffer from the ringing artifact, the quality of fit to the regression model is usually not good. By making a modification of the regression model for such DCT blocks, we can also obtain a good perceptual view. The proposed algorithm does not employ any sophisticated edge-oriented classifiers and nonlinear filters. Compared to the previously proposed algorithms, the proposed algorithm provides comparable or better results with less computational complexity.

Motion compensated frame interpolation by new block-based motion estimation algorithm

In this paper, a motion compensated frame interpolation (MCI) algorithm adapting new block-based motion estimation (BME), which is overlapped block-based motion estimation (OBME), is proposed for frame rate up-conversion (FRC). Unlike conventional BME algorithms, where a video frame is divided into many non-overlapping square blocks of pixels, the proposed BME is executed using an overlapped matching block in order to get more accurate motion trajectory. To reduce computational complexity caused by the OBME, a sub-sampled pixel block is used for the OBME. The proposed OBME is executed with the various overlapped blocks that have different block sizes and sub-sampling ratios. In this paper, instead of mean absolute difference (MAD) used for general BME, a modified MAD is applied for the OBME. The MAD proposed is weighted using the magnitude of MV that corresponds to the estimating position. From the simulation results, the proposed methods show better performance in estimating "true motion" and reducing blocking artifacts with less complexity in calculation compared to a conventional method.

Postprocessing technique for blocking artifacts reduction in DCT domain

An effective post-processing technique is proposed to reduce the blocking artifacts in the block discrete cosine transform (BDCT)-coded images. Simulation results indicate that the proposed scheme outperforms conventional post-processing techniques in both PSNR and visual quality.

Blocking artifact reduction in compressed images based on edge-adaptive quadrangle meshes

During image deblocking, the preservation of visually significant edges in the image is important. Based on the observation that blocking artifact along edge direction is difficult for human perception, a number of existing techniques have proposed to filter the edge pixels with low weight or bypass them altogether to avoid blurring. However, the un-processed artifacts that are anti-parallel to the edge direction often fragment the edges and seriously degrade visual quality. In this paper, by considering the behavior of intensity evolution along and across edges, we propose a new POCS-based algorithm using a new smoothness constraint set. The new constraint set is realized indirectly by limiting the distance between the coded image and the image simulated using edge-adaptive quadrangle meshes. Experimental results indicated that our method compares favorably with several existing techniques.